1. Field of the Invention
The present invention relates generally to optics, and in particular, relates to an apparatus and method for correcting aberrations in holographic optical elements, such as those used in photon collection devices.
2. Background Information
Holographic optical element (HOE) devices can be used to replace bulky and expensive traditional optical components made of glass and plastics. For example, HOE devices can be designed to perform reflecting (to replace mirrors), beam splitting (to replace beam splitters), and collimating or focusing functions (to replace lenses) in an optical system. Typically, the interference pattern of at least two optical wavefronts is recorded in a photosensitive media to produce the amplitude and/or phase modulation needed for the HOEs. Because HOEs are typically fabricated on/in the thin layer of a photosensitive media on a thin substrate, the weight and size of the elements are greatly reduced compared to their refractive counterparts. The reduced size of optical components enables a more compact optical system package.
Due to the practical complications of recording HOEs at infrared (IR) wavelengths, HOEs are generally recorded at visible wavelengths, thereby leading to spherochromatic aberration effects which dominate performance when playing back spherically constructed HOEs at IR wavelengths. Typically, the exposure geometry used in creating the HOEs results in spherical aberrations or other aberrations, such as astigmatism, chromatic aberration, coma, curvature of field, or distortion during playback. Spherical aberration results in a large spot size at the conjugate and is difficult to correct in extremely fast, space-limited optical systems. Accordingly, there is a need for improvements in the correction of spherical aberrations or other aberrations in optical systems incorporating HOEs.
According to an aspect of the invention, an apparatus includes a first element having first and second surfaces. The first surface is positionable to face incident light rays. An emulsion material is disposed over the second surface of the first element and has recorded thereon an interference pattern. A second element has a first surface disposed over the emulsion material and has an aspheric second surface to correct aberrations associated with resulting light rays derived from the incident light rays diffracted by the interference pattern.